Doubt occurs naturally to anybody who thinks. If we think upon matters of ultimate concern we will have big doubts. For this reason, many religious groups advocate that one should not doubt and this leads them commonly to also advocate that one should not think.
Personally, I do think and I think that advocating not thinking is not just dangerous but oppressive. Thought has its place and with it comes doubt. Now the non-specialist may think that what doubt indicates is "a debilitating worry about the meaning of taking a certain course of action that can end up in miserable inaction". This is true. Doubt can be extremely debilitating. In Zen this is called koan work. It is not just a matter for the non-specialist. Traditionally the koan should reach such an intensity that it is like an iron ball stuck in the jaws of a fish - the fish cannot spit it out, but cannot swallow it either. Faith is, inter alia, the faith to face out doubt - to relish it even. Even in the Theravada texts the Buddha says that there is no day as good as a day of striving. This is surely what he means. We live in the constant uncertainty of not knowing who or what we are or where we are going, yet we cannot help the up-surging will to meaning - we want to know. In spiritual traditions, this is equivalent to the dilemma of not knowing if one is saved or not. In Pureland, one chants the nembutsu with the faith that every single nembutsu has the power to save you and with the certainty that you cannot know whether you are, were or will be saved no matter how many nembutsu you utter, yet it matters more than anything else. Buddhism, not just Pureland, says that all confected things (samskaras) are impermanent, but it does not say that everything falls within that category. The Buddha also talks about "the Unborn". By definition, however, the Unborn cannot be contrived.We exist so we must do something with our life and so we have a craving for meaning, but we can never have certainty. Religiously, this means that we can conceive the divine but we cannot coerce it. The history of religion is full of efforts to do so which is quite understandable because people want to reduce the uncertainty and its concommitant anxiety and doubt. The locus of doubt, however, is also the point at which a person is most vibrantly alive.